On the origin of binaries with twin components

The existence of a statistically significant excess of dwarf binaries with mass ratios from 0.95 to 1, as first noted by Lucy & Ricco (1979), is confi rmed by modem data. Excess of such binaries, called twins, is found only fo r periods shorter than 40 days, and in the 2-30 days period range they cons titute 10-20% of the total binary population. Twins must have been formed b y a special mechanism. It cannot be a mass transfer in contact pre-main-seq uence binaries, fission is also unlikely. As shown by Bate (2000), accretio n onto a close binary shifts mass ratio towards 1, so twins may originate f rom binaries that became close while still surrounded by massive envelopes. Many twins are members of higher-multiplicity systems and/or clusters, whi ch probably explains an early formation of a close binary by stellar dynami cs.